The present disclosure relates to the field of interface devices comprising a plurality of sensors and/or actuators.
In many cell-physiological and medical applications, there is a desire to measure biochemical parameters in the close vicinity of living cells.
A popular neurons-on-chip technique is the Multi-Electrode Array (MEA). MEAs are made of an insulating substrate (e.g. glass), with a maximum of around 100 electrodes and a minimal electrode pitch of around 10 μm. MEAs do not contain active sensors, so all electrodes have to be connected to an external amplifier.
In order to perform experiments on large networks of hippocampal neurons, pitch sizes (representing the distance between two neurons) of less than 10 μm and array sizes of more than 10K sensors have to be used. Such experiments call for the use of active sensor arrays.
More recent designs are made of a few planar sensors based on ISFET-transistors. An ISFET-transistor is a MOSFET-transistor wherein the gate electrode is replaced by an electrolyte solution, enabling the transistor to sense charges present at the oxide surface.
An active large sensor array is developed in ‘A 128×128 CMOS Biosensor Array for Extracellular Recording of Neural Activity’ (Eversmann B. et al., IEEE J. Solid-State circuits, pp. 2306-2317, December 2003). A 128×128 matrix of sensors based upon OSFET (Oxide-Semiconductor Field Effect) transistors is presented. The sensors have a pitch of 7.8 μm and the chip can support a total multiplexed sampling rate of 32 MS/s, enabling the read-out of the whole array in 500 μs which is equal to a frame rate of 2K frames per second.
A large analogue readout circuit can raise matching problems: two sensor circuits located a certain distance apart can show different amplification factors and bias offsets. The sensors produce signals with very low amplitudes, which impose noise problems. Further, the sensors measure a signal with respect to a common reference electrode, whereby the signal comprises the signal of interest, noisy signals from neighbouring cells, and interference from the environment.